1993 940 NA - Rex Regina

Couple weeks ago went for NJ emissions testing Failed NOx at 3300 ppm, HC, CO fine. After the test, when accelerator full down, car bogged, and if held, would die, However, everything OK if let up a little on accelerator. Car had new aftermarket cat last fall (never emission tested thought) and has sat in driveway for last 3 months without driving. Replaced air filter, cleaned out air box, ran fuel system cleaner through it, and drove 1000 miles. No more full acceleration problem

Tried again. Failed. NOx 2000ppm, HC up slightly to .89 (spec is .7). Again, after the test, car bogged under full flat out acceleration. (If you have a NA 940, you know you have to floor it to get up to parkway speed!) Did it one time. Check for Codes - None.

Is it possible for O2 sensor to cause high NOx? I noticed my gas mileage is down to around 22 highway. In the past I've gotten 25 mpg.

So, I'm thinking about putting in new plugs.
Then, replacing O2 sensor, (is the O2 Senor test in the FAQ reliable?)
Then, after all that, replacing the cat. (Still under warranty)

Does that seems reasonable? Or should I just go right ot a new cat?

Thanks,

Tom